Abstracts Volume IAVCEI – 5IMC, Querétaro, México MAAR Contrasting eruptive styles of late Pleistocene-to-Holocene monogenetic volcanism from maars to domes in the Serdán-Oriental basin, eastern Mexican Volcanic Belt Post-meeting field guide for the 5th International Maar Conference, Querétaro, México Gerardo Carrasco-Núñez1, Michael H. Ort2, Nancy R. Riggs2, Brian Zimmer3, Lorena De León-Barragán1, Mario López-Rojas1 1 Centro de Geociencias, Universidad Nacional autónoma de México, UNAM Campus Juriquilla, Querétaro, Qro., 76100 México. 2 Northern Arizona University, Flagstaff, AZ, USA. 3 Appalachian State University, Boone, NC, USA. November 23-25, 2014 173 IAVCEI – 5IMC, Querétaro, México Abstracts Volume Cover description: Cerro Pinto, photograph showing unconformable bedding planes at the hinge of the northern tuff ring. Photo by: Gerardo Carrasco-Núñez. Universidad Nacional Autónoma de México Centro de Geociencias Querétaro, Mexico November 2014 Edition and design: J. Jesús Silva Corona 174 Field Trip Guide IAVCEI – 5IMC, Querétaro, México Contrasting eruptive styles of late Pleistocene-to-Holocene monogenetic volcanism from maars to domes in the Serdán-Oriental basin, eastern Mexican Volcanic Belt Gerardo Carrasco-Núñez1,*, Michael H. Ort2, Nancy R. Riggs2, Brian Zimmer3, Lorena De León-Barragán1, Mario López-Rojas1 1 Centro de Geociencias, Universidad Nacional autónoma de México, UNAM Campus Juriquilla, Querétaro, Qro., 76100 México. 2 Northern Arizona University, Flagstaff, AZ, USA. 3 Appalachian State University, Boone, NC, USA. * [email protected] INTRODUCTION canoes, starting with Alchichica basaltic crater where we will see the transition from strombolian to phreato- The purpose of this field guide is to describe a magmatic activity. Then we will explore Cerro Pinto three-day itinerary to examine examples of recent (late tuff-ring-dome complex volcano, looking at evidence Pleistocene and Holocene) monogenetic volcanism of alternating periods of explosive and effusive activ- in the eastern Trans-Mexican Volcanic Belt (TMVB; ity that produced a complex stratigraphy. After that, Figure 1) with particular emphasis on the phreatomag- we will examine the internal structure of the basaltic matic structures (maar-type volcanoes). In this area, Atexcac maar volcano, which records strong interac- these include spectacular exposures of deposits from tions with the country rock and vent migration. At the a diversity of eruptive styles and contrasting composi- final stop of the day, we will examine pyroclastic se- tions. The young age of some of these volcanoes and quences of the Tepexitl rhyolitic tuff-ring. the evolution of some others may be relevant for vol- On day 3, we will visit Tecuitlapa and Aljojuca ba- canic hazard assessments within the area. We have se- saltic maars, where we will discuss the influence of lected the most outstanding volcanoes that have easy the regional tectonic stress regime on the formation access for this field trip. of aligned maar and cinder cones and the consequent In this field trip, we will focus on maar volcanoes, migration of the eruptive loci following these structur- which in Mexico are collectively named xalapazcos, al systems, as well as how apparent local hydrologic a Nahuatl term meaning “vessel with sand”, conditions (Toba café aquifer) favor the formation of whereas those craters with an interior lake are called dry or wet volcanoes. axalapazcos. We will have the opportunity to compare different landforms and eruptive products derived from both basaltic and rhyolitic magmas, producing REGIONAL GEOLOGIC SETTING basaltic maar volcanoes sensu stricto (where the volcano excavated down into the country rock) such The Serdán-Oriental Basin (SOB) and the Cofre as Atexcac or Aljojuca craters, or Tecuitlapa where de Perote-Citlaltépetl volcanic range comprises the a notably well-developed migration of the eruptive Eastern Mexican Volcanic Belt (EMVB). This prov- locus is observed, a rhyolitic tuff-ring like Tepexitl or a ince is characterized by Neogene to Holocene E-W- rhyolitic tuff ring-dome complex such as Cerro Pinto, trending volcanism that goes from the Pacific coast to and a large basaltic maar evolving from a strombolian the Gulf of Mexico covering a large area of Central scoria cone at Alchichica. Mexico (Figure 1). Typical aligned large stratovolca- We will also visit a vitrophyric rhyolite dome noes form N-S high volcanic ranges that are cut by (Cerro Pizarro) and Holocene lava flows derived from large intramontane lacustrine basins such as the SOB Los Humeros caldera whose implications for hazard in the EMVB. Also, large monogenetic fields includ- assessment will be discussed. The locations of the vol- ing thousands of scoria and lava cones and maar vol- canoes are shown in Figure 1. canoes, large caldera volcanoes, several rhyolite dome On Day 1, after a 4-hour drive to the Serdán-Ori- volcanoes, and large silicic calderas form important ental Basin, we will explore avalanche-related depos- volumes of recent volcanism. Composition of the its and explosive products associated with the activity magmas is dominantly calc-alkaline, ranging from ba- of the Cerro Pizarro rhyolitic dome and discuss the salt to rhyolite. polygenetic origin of what was previously regarded The SOB is dominated by monogenetic bimodal as an isolated monogenetic rhyolitic dome. After that, volcanism of Quaternary age, with a minor propor- we will visit rim-caldera basaltic lavas derived from tion of isolated Pliocene volcanoes (Negendank et Los Humeros complex and we will pass the Cantona al. 1985). Basement Cretaceous limestone rocks and archaeological site, which is regarded as one of the Tertiary intrusive rocks are also exposed in the area largest pre-Hispanic cities in Mexico. (Yáñez and García, 1980), and abundant Miocene On Day 2, we will drive to the middle part of the volcanic rocks are exposed to the west of the basin Serdán-Oriental basin to explore different types of vol- (Carrasco-Núñez et al., 1997). 1 IAVCEI – 5IMC, Querétaro, México Field Trip Guide Figure 1. Location of the field trip area in the eastern Trans-Mexican Volcanic Belt (A), and Digital Elevation Model (B) showing the morphology and distribution of the main volcanoes to be explored in the field trip, including Los Humeros caldera, Cerro Pizarro rhyolitic dome, Alchichica crater, Cerro Pinto tuff ring-dome complex, Atexcac maar, Tepexitl tuff-ring, Aljojuca maar and Tecuitlapa maar. Basement and intrusive rocks younger volcanism of the EMVB, but thicken to the north, forming the Sierra Madre Oriental province, Highly deformed Cretaceous limestone and shale a large NW–trending folded and faulted mountain form the regional basement of the EMVB. Palaeozo- range. Isolated small Oligocene and Miocene plutons ic crystalline basement are exposed in the Teziutlán of granodiorite, monzonite and syenite intrude the massif, northeast of Los Humeros volcano (Vinieg- basement rocks (Yáñez and García, 1982). The oldest ra, 1965). These basement rocks are covered by the exposed volcanic rocks in the SOB are andesitic la- 2 Field Trip Guide IAVCEI – 5IMC, Querétaro, México vas of Pliocene age (3.5 Ma, Yañez and García, 1982); (Carrasco-Núñez, 2000) (Figure 1). This range rep- however, voluminous Miocene andesitic rocks form resents an important physiographic divide separating the western range of the SOB (Carrasco-Nuñez et al., the Altiplano (Serdán-Oriental basin) to the west from 1997; Gómez-Tuena and Carrasco-Núñez, 2000). the Gulf Coastal Plain to the east, with a difference in relief of about 1 km, which in addition with the slop- ing substrate to the east promoted unstable conditions Serdán-Oriental Basin of the large volcanic edifices forming that range and repeated flank collapses toward the Gulf Coastal Plain The SOB is characterized by monogenetic bi- (Carrasco-Núñez et al., 2006). modal (basaltic andesite and rhyolite) volcanism rep- Although Citlaltépetl (Pico de Orizaba) stratovol- resented by isolated cinder and lava cones of basaltic cano is regarded as the only active volcano within the composition, scattered rhyolitic domes and maar vol- eastern Trans-Mexican Volcanic Belt, other volcanic canoes, tuff rings and tuff cones. The basin is a broad, activity have occurred in Holocene times such as the internally drained high plain (2,300 m a.s.l.), bounded pre-Columbian eruption of El Volcancillo at the north- to the north by the ~20 km-diameter Los Humeros ern flanks of Cofre de Perote (Siebert and Carrasco- caldera, which has been active since ~0.46 Ma. Vo- Núñez, 2002) and the recent reports of caldera-rim luminous caldera-forming eruptions produced widely lavas from Los Humeros volcanic complex (Dávila- distributed pyroclastic ignimbrites and fall deposits Harris and Carrasco-Núñez, 2014) and Aljojuca crater that cover the northern part of the Serdán-Oriental (De León and Carrasco-Núñez, 2014). basin. The western part of the SOB forms a volcanic range comprising the Miocene andesitic Cerro Grande volcano and the Pleistocene andesitic-dacitic La Ma- VOLCANISM OF THE linche stratovolcano, while the eastern edge is formed SERDÁN-ORIENTAL BASIN by the Citlaltépetl-Cofre de Perote range (Figure 1), comprising mainly Quaternary andesitic stratovolca- Los Humeros Caldera noes where Citlaltépetl volcano remains active with passive fumarolic activity. Los Humeros volcano is one of the largest calderas The SOB includes more than 10 maar volcanoes, in central Mexico and hosts one of the most important located in the middle